In general, compounds which are to be distributed onto substrates and pyrolyzed to form metal oxide coatings can be applied in different forms: vapor, liquid or solid.
The applicant has developed a technique of distributing such pyrolyzable compounds in a solid form, which presents considerable advantages compared to the other techniques which use vapors or liquids. This mode of distribution provides very high-performance results which, further, are particularly well suited to the formation of thin layers on a glass ribbon produced according to the float process. According to this process, the glass is produced in the form of an endless ribbon, in a width on the order of 3.5 m at a speed between 6 and 24 m/min, depending upon the specific thickness produced, and which most often is spread between 12 and 18 m/min considering the thicknesses most frequently marketed.
To be able to provide good results in the coating techniques using compounds in powder form, such powdered compounds must exhibit the determined characteristics, in particular the morphological characteristics (shape of powder grains, flowability, and the like).
Actually, for questions of rapid deposition, but also from the aspect of the quality of the coating layer, the powder grains must reach the substrate with a certain energy, therefore with a certain speed, as well as a certain weight. These grains must not, however, be too heavy for fear of leading to a series of defects risks of spots due to the explosion of too coarse grains caused by the fact of their great energy up to flames that burn at the output of the float installation, risks of blemishes due to the distinction of individual impacts of each grain, and the like.
Further, the powder must be easily transportable through all metering and distribution devices which deliver it perpendicularly to the ribbon of glass For this reason, certain forms of powder grains are better suited than others. When the metal oxide layers are on the order of a micron thick, which generally produces interference layers, it is essential that the distribution of the powder on the substrate be perfectly uniform and constant, otherwise irregularities of thickness of the coating layer results, such irregularities being evident by the local variations of properties, and in particular by variations of color visible to the eye, which thus makes the coated substrate unmarketable.
Therefore it is essential that the morphological characteristics of the produced powder be controllable.
It is known, in particular by the document of European patent 192,009, how to produce indium formate according to the following reactions:
In+3HCl.fwdarw.InCl3+3/2 H2
InCl3+3NH4OH.fwdarw.In(OH)3+3NH4Cl
In(OH)3+3HCOOH.fwdarw.In(HCOO)3+3H2O
This process is generally satisfactory, but the number of parameters brought into play is such that it allows the control of the morphology of the produced powder only with difficulty and then with a series of additional reactions which are both costly and time consuming. As previously mentioned, the controlled morphology powder exhibits the greatest interest for the process of pyrolysis of such powders on heated substrates, in particular, glass.
Further, this production necessitates a continual intervention, it is a consumer of large quantities of reagents, it requires a large installation and it is discontinuous.